J. H. van Helden

Direct and wavelength modulation spectroscopy using a cw external cavity quantum cascade laser

A continuous wave external cavity quantum cascade laser (EC-QCL) operating between 1872 and 1958 cm−1 has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the ν2 band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of 8.4×10−4 cm−1 Hz−1/2 was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of 23 to 3.7×10−5 cm−1 Hz−1/2.

Sensitive trace gas detection with cavity enhanced absorption spectroscopy using a continuous wave external-cavity quantum cascade laser

Trace gas sensing in the mid-infrared using quantum cascade lasers (QCLs) promises high specificity and sensitivity. We report on the performance of a simple cavity enhanced absorption spectroscopy (CEAS) sensor using a continuous wave external-cavity QCL at 7.4 μm. A noise-equivalent absorption coefficient αmin of 2.6 × 10–8 cm–1 in 625 s was achieved, which corresponds to a detection limit of 6 ± 1 ppb of CH4 in 15 millibars air for the R(3) transition at 1327.074 cm–1. This is the highest value of noise-equivalent absorption and among the longest effective path length (1780 m) reported to date with QCL-based CEAS.

Application of quantum cascade lasers in studies of low-pressure plasmas: Characterization of rapid passage effects on density and temperature measurements

The application of quantum cascade lasers in the intrapulse operation mode for low-pressure plasma spectroscopy is hampered by the observation of rapid passage effects, leading to lower quantitative accuracy. We demonstrate that accurate densities and rotational temperatures of CH4 within a CH4 plasma can be obtained by characterizing the rapid passage effects in gas phase conditions prior to carrying out the plasma measurements. Furthermore, we show that the ratios of the integrated absorption of two transitions are not affected by the rapid passage effect and, thus, rotational temperatures of species can be obtained.

Characterization of an external cavity diode laser based ring cavity NICE-OHMS system

The performance of an external cavity diode laser based noise immune cavity enhanced optical heterodyne molecular spectrometer is presented. To reduce the noise on the signal a ring cavity and a circuit to remove residual amplitude modulation on the pre-cavity laser radiation was implemented. We demonstrate a sensitivity of 4 x 10(-11) cm(-1) Hz(-1/2) using a cavity with a finesse of 2600 on a Doppler-broadened transition of CH(4) at 6610.063 cm(-1).

Time-Resolved Detection of the CF3 Photofragment Using Chirped QCL Radiation

This paper demonstrates how a quantum cascade laser (QCL) in its intrapulse mode can provide a simple method for probing the products of a photolysis event. The system studied is the 266 nm photodissociation of CF3I with the CF3 fragments subsequently detected using radiation at approximately 1253 cm(-1) generated by a pulsed QCL. The tuning range provided by the frequency down-chirp of the QCL operated in its intrapulse mode allows a approximately 1 cm(-1) segment of the CF3 nu3 band to be measured following each photolysis laser pulse. Identification of features within this spectral region allows the CF3 ( v = 0) number density to be calculated as a function of pump-probe delay, and consequently the processes which populate and deplete this quantum state may be examined. Rate constants for the population cascade from higher vibrational levels into the v = 0 state, k 1, and for the recombination of the CF3 radicals to form C2F6, k2, are measured. The returned values of k1 = (2.3 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) and k2 = (3.9 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) are found to be in good agreement with reported literature values.

Rapid passage effects in nitrous oxide induced by a chirped external cavity quantum cascade laser

A widely tunable pulsed external cavity quantum cascade laser operating around 8 μm has been used to make rotationally resolved measurements of rapid passage effects in the absorption spectrum of N2O. Rapid passage signals as a function of laser power and N2O pressure are presented. Comparisons are drawn with measurements performed on the same transition with a standard distributed feedback quantum cascade laser. The initial observations on rapid passage effects induced with an external cavity quantum cascade laser show that such high power, widely tunable radiation sources may find applications in both nonlinear optics and optical sensing experiments.

Rapid passage signals from a vibrationally excited target molecule: a pump and probe experiment with continuous wave quantum cascade lasers.

Two 5 µm continuous wave quantum cascade lasers are used to perform a counterpropagating pump and probe experiment on a low pressure sample of nitric oxide. The strong pump field excites a fundamental rovibrational transition and the weaker probe field is tuned to the corresponding rotationally resolved hot band transition. When both light fields are in resonance, rapid passage is observed in the hot band absorption lineshape arising from a minimally damped and velocity-selected sample of molecules in the v=1 state. The measured rapid passage signals are well described by a two-level model based on the optical Bloch equations.

Rapid passage signals induced by chirped quantum cascade laser radiation: K state dependent-delay effects in the nu(2) band of NH3

In this Letter, a 10 microm quantum cascade laser operating in the intrapulse mode is used observe rapid passage (RP) effects within a 40 cm single-pass gas cell containing low pressures of NH(3). The laser tuning range allows the rotational states J=2 with K=0, 1, and 2 to be probed. We show that the RP structures change as a function of optical density and that the magnitude of the delay in the switch from absorption to emission as a function of increased gas pressure is dependent upon the initial value of K. These measurements are qualitatively well modeled using the Maxwell-Bloch equations.

A 3 µm difference frequency laser source for probing hydrocarbon plasmas

The practicality of a compact solid-state laser-based difference frequency generation system is demonstrated as a tool for probing hydrocarbon-based plasmas. The laser light of a cw Nd : YAG operating at 1064 nm and one of two distributed feedback diode lasers operating at 1560 and 1620 nm were mixed in a periodically poled lithium niobate crystal producing mid-infrared radiation at 3.35 and 3.1 µm for the detection of CH4 and C2H6 at the first wavelength and C2H2 and C2H4 at the latter. The radiation was used to probe a rf capacitively coupled CH4 plasma for a matrix of conditions, varying power (<180 W) and pressure (<1 Torr) in both direct absorption spectroscopy and wavelength modulation spectroscopy (WMS) experiments to achieve relatively high sensitivities while retaining accurate spectral information in the form of linewidths. A minimum detectable absorption coefficient αmin of 2 × 10−5 cm−1 was achieved for direct absorption spectroscopy which corresponds to a minimum detectable density for CH4 of 1.7 × 1012 cm−3 and an αmin value of 2 × 10−6 cm−1 was achieved for WMS measurements on C2H2. The depletion of CH4 was measured to increase from 23% at 40 W to 40% at 180 W at a pressure of 0.84 Torr, and the depletion increased to 56% as the pressure decreased to 0.45 Torr.